1. Field of the Invention
The present invention relates generally to chemical mechanical polishing (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to a wafer carrier utilized in a subaperture CMP system.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform CMP operations, including polishing, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
In a typical CMP system, a wafer is mounted on a carrier, which rotates in a direction of rotation. The CMP process is achieved when the exposed surface of the rotating wafer is applied with force against a polishing pad, which moves or rotates in a polishing pad direction. Some CMP processes require that a significant force be used at the time the rotating wafer is being polished by the polishing pad.
Normally, the polishing pads used in the CMP systems are composed of porous or fibrous materials. However, in some CMP systems, the polishing pads may contain fixed abrasive particles throughout their surfaces. Depending on the form of the polishing pad used, a slurry composed of an aqueous solution such as NH4OH or DI water containing dispersed abrasive particles may be applied to the polishing pad, thereby creating an abrasive chemical solution between the polishing pad and the wafer.
Several problems may be encountered while using a typical CMP system. One recurring problem is called xe2x80x9cedge-effect,xe2x80x9d which is caused when the CMP system polishes the edge of the wafer at a different rate than other regions, thereby causing a non-uniform profile on the surface of the wafer. The problems associated with edge-effect can be divided to two distinct categories. The first category relates to the so-called xe2x80x9cpad rebound effectxe2x80x9d caused as a result of the initial contact of the polishing pad with the edge of the wafer. The second category will be described below.
FIG. 1A is an illustration of the pad rebound effect associated with the prior art. A wafer 202 is mounted on a carrier 100. Subsequently, the wafer 202 is applied against the pad surface 102 with a force F to accomplish a CMP process. At a certain point in time, the pad surface 102 contacts the edge of the wafer 202 at an edge contact zone 104c, and the pad surface is shown bouncing off the edge of the wafer, thereby creating a non-contact zone 104a. Thereafter, the pad surface comes into contact with the wafer 202 at a contact zone 104b. However, the pad surface 102 bounces off the surface of the wafer 202 again, so as to create another non-contact zone 104a. Then, once more the pad surface comes into contact with the wafer 202 at another contact zone 104b. However, it bounces off again. Thus, the regions of the wafer 202, which came into contact with the pad surface 102 like the contact zones 104b, are polished more than other regions. As a result, the CMP processed wafer will tend to show a non-uniform profile.
The xe2x80x9cburn-offxe2x80x9d effect, which constitutes the second category of problems associated with the edge-effect is shown in FIG. 1B. As illustrated, the burn-off effect occurs when the sharper edge of a wafer 202 is excessively polished as it makes contact with the pad surface 102 (e.g., at the edge contact zone 104c). This happens because a considerable amount of pressure is exerted on the edge of the wafer 202 as a result of the surface pad 102 applying the force F on an infinitely small contact area defined as the edge contact zone 104c. As a consequence of the burn-off effect, the edge of the resulting polished wafers exhibit a burn ring that renders the edge region unusable, thereby wasting silicon device area.
Another shortcoming of conventional CMP systems is their inability to polish the surface of the wafer 202 along a desired finishing layer profile. Ordinarily, the surface of a wafer 202 that has undergone some fabrication tends to be of a different thickness in the center region and varies in thickness out to the edge. As illustrated in FIG. 1C-1, in a typical conventional CMP system, the pad surface 102, which covers the entire wafer surface, is designed to apply a force on a finishing layer 202a surface. As a result, all the regions of the finishing layer 202a are polished until the finishing layer 202a is substantially flat. Thus, as shown in FIG. 1C-2, the pad surface 102 polishes the finishing layer 202a, irrespective of its wavy profile, thereby causing the thickness of the finishing layer 202a to be non-uniform (i.e., at points 202a1, 202a2, 202a3, and 202a4). As is well known, some circuit fabrication applications require that a certain thickness of material be maintained in order to build a working device. For instance, if the finishing layer 202a were a dielectric layer, a certain thickness would be needed in order to define metal lines and conductive vias therein.
In view of the foregoing, a need therefore exists in the art for a chemical mechanical polishing system that enables precision and controlled polishing of specifically targeted wafer surface regions, while substantially eliminating damaging edge-effects, pad rebound effects, and edge burn-off effects.
Broadly speaking, the present invention fills these needs by providing a system which implements precision controlled polishing of layer surfaces of a wafer. In one implementation, the CMP system can be made to follow the topography of the layer surfaces of the wafer so as to create a layer surface, which has a uniform thickness throughout. In a preferred embodiment, the CMP system is designed to implement a rotating carrier in a subaperture polishing configuration, thereby eliminating the above-mentioned drawbacks, edge-effects, pad rebound effects, and edge burn-off effects. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a chemical mechanical polishing (CMP) system is disclosed. The CMP system includes a carrier, which has a top surface and a bottom region. The top surface of the carrier is designed to hold and rotate a wafer having a one or more formed layers to be prepared. Further included is a preparation head, which is designed to be applied to at least a portion of the wafer that is less than an entire portion of the surface of the wafer.
In another embodiment, a chemical mechanical polishing (CMP) system is disclosed. A carrier is designed to hold and rotate a substrate at a fixed location. The carrier includes a surface to be prepared. Also included is a primary head. The primary head is designed to move over the fixed location of the rotating carrier in one of a first direction or a second direction. A first direction begins at about the center of the surface to an edge of the surface, and a second direction begins at about the edge of the surface to about the center of the surface. The primary head is further designed to be applied to at least a portion of the surface, which is less than an entire portion of the surface.
In still a further embodiment, a chemical mechanical polishing (CMP) system is disclosed. The CMP system includes a carrier having a top surface and a bottom region. The top surface of the carrier is designed to hold and rotate a wafer, which has one or more formed layers to be prepared. Also included is a preparation head designed to be applied to at least a portion of the surface of the wafer that is less than an entire portion of the surface of the wafer. Further included is a conditioning head positioned beside the carrier. The conditioning head is designed to have a conditioning surface that is substantially planer with a top surface of the wafer. The conditioning head is further designed to deliver the preparation head as the preparation head is moved onto the top surface of the wafer. Alternatively, the conditioning head is designed to receive the preparation head as the preparation head is moved onto the conditioning head.
The advantages of the present invention are numerous. Primarily, rather than polishing all the regions of the surface of a wafer until the surface of the wafer is substantially flat, the subaperture CMP system, precisely and controllably, polishes specifically targeted wafer surface regions. Thus, in one embodiment, the CMP system can be made to follow the topography of the finishing layer, thereby creating a finishing layer having a uniform thickness throughout. In addition, the subaperture configuration of the CMP system in conjunction with the carrier implemented, substantially eliminate the edge-effects, pad rebound effects and edge burn-off effects associated with the prior art. Further advantages associated with the subaperture CMP, system include, without limitations, substantially lower footprint, machine volume, and cost of ownership.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.